Sébastien Comas-Cardona

Gas transport in fibrous media: Application to in-plane permeability measurement using transient flow

This article introduces a methodology to measure in-plane permeability of fibrous media using a transient one-dimensional air flow with absolute pressures ranging from 103 to 105 Pa. The method, based on the measurement of gas pressure at the boundaries throughout the transient flow, is convenient, clean and fast, avoids usage of a gas flow meter and offers a way to study the gas transport within fibrous media. The transport of a compressible fluid is described by several models to comply with different flow regimes which can occur during the experimental measurements. A thermal analysis is given to verify the validity of isothermal conditions during the tests. The permeability, only depending on the fibrous structure, is determined by inverse method, fitting the simulation results to the experimental data obtained using raising or dropping pressure methods. The deviation from Darcys law caused by gas sliding effect is analysed and a relative parameter of fabric material shows a dependence in permeability, with a similar trend as the Klinkenberg sliding parameter in soils and rocks.

Efficient experimental characterisation of the permeability of fibrous textiles

Two experimental set-ups used to characterise the in-plane and through-thickness permeabilities of reinforcing textiles have been developed and are presented. Both the experimental testing and data processing techniques used have been selected to ensure that the characterisation is completed in an efficient and robust method, increasing the repeatability of tests while minimising user induced errors as well as the time and resources needed. A number of key results and outputs obtained are presented from tests carried out on a plain woven reinforcing textile with a range of number of layers and at different fibre volume fractions.

Experimental Characterization and Modeling of Bending Properties of Woven Fibrous Preforms

The properties of final composite parts depend on properties of dry preforms often being formed over doubly-curved shapes. In this case the fibrous preforms exhibit intricate large deformations, including shear, tension, and bending modes. Although the bending stiffness of fibrous materials is small, in shaping of preforms, when wrinkling occurs, its influence is important, not negligible and responsible for the wrinkles shape. Because of the structural and mechanical peculiarities, the experimental determination of bending properties of fibrous materials is rather complex, and there is no unique generally adopted test. A set of cantilever tests was chosen to be carried out in this study, in the form of sequence of different loading cases for one material that permits to reveal the eventual non-linear and non-elastic behavior of the material in bending. The tests were realized for glass fabrics with different types of weaving patterns and different areal weights. The effect of these parameters on the bending response is studied. The analysis of the data on bending of fabrics and bending of yarns, extracted from fabrics with the preserved undulated shape, is performed as well. The regions of the deformed specimens characterized by the largest scatter of experimental data are identified and analyzed. Besides, the analytical model based on corrugated plates theory, taking into account the undulated architecture of fabrics, is employed to characterize its bending properties, and to make a future comparison with the test results.

Three-dimensional mechanical properties of dry carbon fiber tows subjected to cyclic compressive loading:

Characterizing the response of compressed dry fibrous reinforcements is a key feature to control liquid composite molding processes. Considering preforms manufactured by automated dry fiber placement, supplementary information are needed, since cyclic loading occurs, during which the mechanical properties of tows evolve. In this way, an extensive characterization procedure is proposed in this article. Each mechanical feature, such as nonlinearity, springback, irreversibility, stabilization, and strain rate dependence, is analyzed, and when possible, related to microstructural observations. Particular attention is paid to the response in other directions than the compressed one. Finally, from this characterization, a route to modeling is drawn: analogies with other classes of materials are proposed, leading to choices on the modeling of each mechanical property.

Introduction of intra-tow release/storage mechanisms in reactive dual-scale flow numerical simulations

Classical dual-scale reactive simulations of the RTM process assume permanent intra-tow resin storage in the saturated domain. However, recent experimental investigations revealed that permanent storage is occurring only in a limited volume of the tows. In the remaining volume, fluid is released in the channels with a rate that depends on the architecture of the textile and on the fiber volume fraction. Based on experimental observations, a new model is proposed to refine the simulation of the high speed reactive RTM process: a simplified microstructural model is used to enable permanent and partial transient storage within the tows. Additionally, a new sink term is proposed to reproduce the kinetics of the convective tow-channel fluid exchanges in the saturated domain. After a state of the art on dual-scale and reactive flow, the experimental inputs of the study are presented. The new model is then introduced, validated and characterized using the experimental inputs. Additionally, the influence of the release mechanisms on a reactive dual-scale injection is estimated by conducting comparative single-scale, and dual-scale simulations with transient or permanent storage. The new model has been demonstrated to be appropriate to reproduce accurately the release mechanisms, and simulations reveal the interest of taking these release mechanisms into account to simulate reactive dual-scale injections with an increased accuracy.

Multi-scale modelling of non-uniform consolidation of uncured toughened unidirectional prepregs

Consolidation is a crucial step in manufacturing of composite parts with prepregs because its role is to eliminate inter- and intra-ply gaps and porosity. Some thermoset prepreg systems are toughened with thermoplastic particles. Depending on their size, thermoplastic particles can be either located in between plies or distributed within the inter-fibre regions. When subjected to transverse compaction, resin will bleed out of low-viscosity unidirectional prepregs along the fibre direction, whereas one would expect transverse squeeze flow to dominate for higher viscosity prepregs. Recent experimental work showed that the consolidation of uncured toughened prepregs involves complex flow and deformation mechanisms where both bleeding and squeeze flow patterns are observed [1]. Micrographs of compacted and cured samples confirm these features as shown in Fig.1. A phenomenological model was proposed [2] where bleeding flow and squeeze flow are combined. A criterion for the transition from shear flow to resin bleeding was also proposed. However, the micrographs also reveal a resin rich layer between plies which may be contributing to the complex flow mechanisms during the consolidation process. In an effort to provide additional insight into these complex mechanisms, this work focuses on the 3D numerical modelling of the compaction of uncured toughened prepregs in the cross-ply configuration described in [1]. A transversely isotropic fluid model is used to describe the flow behaviour of the plies coupled with interplay resin flow of an isotropic fluid. The multi-scale flow model used is based on [3, 4]. A numerical parametric study is carried out where the resin viscosity, permeability and inter-ply thickness are varied to identify the role of important variables. The squeezing flow and the bleeding flow are compared for a range of process parameters to investigate the coupling and competition between the two flow mechanisms. Figure 4 shows the predicted displacement of the sample edge with the multi-scale compaction model after one time step [3]. The ply distortion and resin flow observed in Fig.1 is qualitatively retrieved by the computational model.Consolidation is a crucial step in manufacturing of composite parts with prepregs because its role is to eliminate inter- and intra-ply gaps and porosity. Some thermoset prepreg systems are toughened with thermoplastic particles. Depending on their size, thermoplastic particles can be either located in between plies or distributed within the inter-fibre regions. When subjected to transverse compaction, resin will bleed out of low-viscosity unidirectional prepregs along the fibre direction, whereas one would expect transverse squeeze flow to dominate for higher viscosity prepregs. Recent experimental work showed that the consolidation of uncured toughened prepregs involves complex flow and deformation mechanisms where both bleeding and squeeze flow patterns are observed [1]. Micrographs of compacted and cured samples confirm these features as shown in Fig.1. A phenomenological model was proposed [2] where bleeding flow and squeeze flow are combined. A criterion for the transition from shear flow to resin b...

Limit of adhesion coefficient measurement of a unidirectional carbon fabric

The construction of mesoscopic models of fiber washing effects during the high pressure resin transfer molding (RTM) process with thermoplastic polymers requires the knowledge of the limit of adhesion coefficients between dry and impregnated fabric reinforcement plies and the mold. To measure these coefficients, a new method is described. During RTM process, a preform is placed in the cavity. The mold is closed to reach a given cavity thickness. Therefore, the normal stress applied to the preform can be relaxed before injection. In order to comply with these RTM specificities, the limit of adhesion coefficients are measured on relaxed preforms at given thicknesses. Hence, a first set of compression tests followed by a relaxation step are performed in order to measure the normal stresses the preforms undergo. The relaxation can reduce compression normal stress by 30%. The relaxation is taken into account for the limit of adhesion calculation. Then a second setup has been built in order to measure forces during pull-out of fabric plies inserted between other plies or metallic plates. This method provides mean values of coefficients of limit of adhesion for dry and lubricated preforms and various stacking sequences. These coefficients can be refined so as to take into account the real area of the contact interface for ply/ply and ply/mold configurations. There are also useful in the forming process of dry fabrics when inter-ply sliding occurs. Graphical Abstract

Identification and Modeling of Variability in Fabrics Used as Reinforcement in Polymer Composites: Influence on Transport and Mechanical Properties

Variability in fiber architecture and content introduces randomness in transport and mechanical properties of textile reinforcements and composites. Assessment of robustness of both manufacturing processes and composite parts require to link fabric variability to dominant properties. Irregular injection flow patterns or defects in the final products often occur due to the high variability in the fibrous media. Therefore, manufacturing robustness and part reliability have to be studied to avoid trial and error procedures. This study focuses on spatial variability in the fiber volume fraction and architecture and their influence on permeability of fiber reinforcements and mechanical performance of textile composite, relating these important properties to variation in reinforcement architecture. Methods to capture experimentally and model numerically the fabric randomness are presented and illustrated on typical non-woven fabrics. An efficient numerical approach is presented for the simulation of mold filling process with random fibrous permeability as input. Numerical examples for different injection schemes are presented to demonstrate the ability of the current approach in predicting the variability in mold filling results.Copyright

Modeling of Uniaxial Compression of Fiber Reinforcements using Finite Strains

Liquid Composite Molding (LCM) processes are increasingly used to produce composite parts. Most of those processes involve compression of the fiber reinforcement and resin flow. In order to accurately model LCM processes, a good knowledge of fiber reinforcement behavior in compression is required. Several models have already been published, but none of them include permanent deformations. Also because of the large deformation involved in the processes, a finite strain formulation is proposed. Results are given for a glass twill‐weave fabric.